بُلُوغُ الْمَرَامِ
Bulugh al-Maram 386
The Book of Prayer
وَلِأَبِي دَاوُدَ, وَاَلتِّرْمِذِيِّ نَحْوُهُ عَنْ عَائِشَةَ وَفِيهِ: { كُلَّ سُورَةٍ فِي رَكْعَةٍ, وَفِي اَلْأَخِيرَةِ: قُلْ هُوَ اَللَّهُ أَحَدٌ , وَالْمُعَوِّذَتَيْنِ } [1] .
Abu Dawud and at-Tirmidhi report something similar from 'Aishah (RA) that Allah's Messenger (ﷺ) used to recite a Surat in every Rak'at and in the last (third) Rak'at he would recite Qul Huwa Allahu Ahad (Surat al-Ikhlas) and al-Mu'awwidhatain.
Sunnah.com reference: Book 2, Hadith 290